1. Field of the Invention
The present invention relates to a frequency synthesis method as well as the apparatus using this method.
2. Description of the Prior Art
Some transmission systems, in particular those designed to withstand jamming, require very rapid frequency jumps of the carrier, with a known phase so that a coherent demodulation may be provided on reception.
Frequency synthesizers are generators capable of selecting on request a given band from a plurality of available bands of the spectrum.
The output frequency is generally obtained by slaving an oscillator by means of a phase loop to a reference frequency. Frequency switching in the most rapid synthesizers known for frequencies higher than a few MHz, of the phase loop type, requires a minimum time of several microseconds depending in particular on the pass band of the phase loop.
The frequency bands of the spectrum generally result from mixtures, multiplications and divisions of several reference frequencies, and these operations, in particular the digital frequency division, do not allow a phase continuity of the new frequency to be ensured with respect to the old one.
In yet another technique, a frequency comb is produced then the desired spectral band is selected by switching the outputs of a bank of frequency filters. These acoustic surface wave based filters have a not inconsiderable propagation delay, of the order of 1 to 3 microseconds, and counter-balancing the delay time, so the phase shift, forms the limit of this type of synthesizer.
The invention aims at overcoming the insufficiencies and limitations of the above mentioned techniques by providing a synthesizer better able to comply with the high speed switching requirements and continuity of the phase of the signal.